Method and apparatus for controlling air-conditioning systems

ABSTRACT

An air conditioning system controlled for automatically heating and/or cooling a multiple zone building is disclosed wherein each zone includes a zone temperature responsive load transmitter. Each load transmitter produces an output command signal for controlling operation of damper units and heating and cooling equipment. The command signal configuration is modified to avoid operation of heating and cooling equipment throughout a relatively wide &#34;no load&#34; band of zone temperatures centered around the zone set point temperature. The command signal is highly responsive to sensed zone temperature changes beyond the no load band so that the zone temperature tends to be maintained in the no load band. 
     An alternative system is disclosed in which no load bands occur at two distinct command signal values and operation of damper units is effected when the command signal value shifts between the no load band values.

CROSS-REFERENCED PATENTS

U.S. Pat. No. 3,745,778 issued July 1973 to Attridge;

U.S. Pat. No. 3,788,386 issued Jan. 1974 to Demaray;

U.S. Pat. No. 3,915,376 issued Oct. 1975 to Attridge, et al.

BACKGROUND OF THE INVENTION

The present invention relates to air conditioning systems and moreparticularly relates to the control of air conditioning systems whichboth heat and cool an air conditioned zone.

THE PRIOR ART

Air conditioning systems which both heat and cool air conditioned zonesare commonly used and these systems have sometimes employed controlswhich automatically operate the heating or cooling equipment dependingon sensed load conditions in the zones. Prior art control systemsoperated the heating and cooling equipment at relatively narrowtemperature differentials so that zones being controlled were heldclosely to preselected, or "set point" temperatures.

In multiple zone air conditioned buildings, the control systemsfrequently operated the heating and cooling equipment simultaneously sothat while one zone was being cooled, another might be heated, and otherzones at intermediate temperatures might be simultaneously supplied withheated and cooled air. All zones could, in this fashion, be maintainedquite close to the zone set point temperatures. Examples of prior artcontrol systems which function in the manner referred to are disclosedby the above-referenced U.S. patents.

Control systems of the general type referred to were extremelysuccessful in maintaining closely controlled zone temperatures in bothsingle and multiple zone buildings. The zone temperatures were closelycontrolled because the control systems were highly sensitive to zonetemperature changes and reacted promptly to maintain the temperatures atset point levels. This high degree of responsiveness made systemsemploying the controls somewhat expensive to operate. The heating andcooling equipment sometimes tended to be cycled frequently and/oroperated simultaneously. Proposals have been made to reduce the cost ofoperation of the systems by increasing the use of atmospheric air forcooling and providing heating from sources of waste heat (see U.S. Pat.Nos. 3,788,386 and 3,915,376.

As a result of power and fuel scarcity, guidelines concerning thermostatsettings have been adopted which recommended that during the heatingseason, equipment should not be operated until zone temperatures fallbelow 68° F, while cooling equipment should not be operated until thezone temperatures exceed 78° F. These guidelines have been impossible tofollow in many air conditioned buildings of the character referred tobecause zone heating and cooling loads vary widely and control systemdifferentials between heating and cooling have been inherently small. Insuch structures, particularly multiple zone buildings employing theprior art control systems, the upward or downward adjustment of the zoneset point temperatures levels is frequently immaterial so far asoperation of the heating and cooling equipment is concerned because, insome zones, the heating and cooling equipment may be operating togetherto maintain the set point temperature level regardless of where it isset. Consequently, lowering or raising the thermostat settings will notnecessarily permit energy savings.

Guideline-type operation might be approximated by providing controlsystems which are significantly less sensitive and responsive to zonetemperature variations from a set point level but the lack ofsensitivity could permit zone temperatures to fluctuate too widely foroccupants of the buildings to remain comfortable.

SUMMARY OF THE INVENTION

The present invention provides a new and improved method and system forcontrolling heating and cooling equipment in an air conditioning systemwherein the heating and cooling equipment is automatically operated inaccordance with sensed zone temperature conditions, neither the heatingequipment nor the cooling equipment is rendered effective through asubstantial range, or "no load" band, of sensed zone temperatureconditions around the set point temperature and yet the system is highlysensitive and responsive to zone temperatures outside the no load band.

According to preferred embodiments of the invention, an air conditioningsystem having zone heating and cooling equipment is provided with a zonesensor arrangement for producing sensed zone temperature conditionsignals and a command function generator for effecting operation ofeither the heating or the cooling equipment. The command functiongenerator comprises a command signal generator and a command signalmodifier which coact to produce a command signal having first and secondvalue ranges within which the respective heating and cooling equipmentoperates and an intervening no load band region within which the heatingand cooling equipment does not operate even though the zone temperaturemay vary through a substantial range.

The no load band is so called because even though zone temperatureschange within the band these changes are ineffective to signal a changein load on the zone and since the heating and cooling equipment does notoperate when the temperature is in the band.

The command signal values vary substantially within the first and secondvalue ranges as a function of sensed zone temperature conditions inthese ranges. This significant variation in command signal level versussensed zone temperature change assures a high degree of control systemsensitivity and responsiveness to zone temperature levels. The commandsignal modifier affects the command signal generator operation so thatcommand signal values in an intervening no load band region varyrelatively little in response to sensed zone temperature conditionchanges.

The new control system thus insures that the heating and coolingequipment are maintained inactive so long as sensed zone temperatureconditions are within a predetermined range of the set point temperature(i.e., in the no load band), yet provides for prompt and effectiveoperation of zone heating or cooling equipment whenever the sensed zonetemperature conditions are outside of the range. Zone temperatures thuscan vary within a relatively wide no load band without requiringoperation of any air conditioning equipment and operating costs arereduced.

Another important feature of the invention resides in the maintenance ofzone temperatures within a wide no load band by anticipating thedirection of temperature change in the zone during the no load band andconditioning the air conditioning system to oppose such changes withoutoperating the heating and cooling equipment. An illustrated forced air,air conditioning system utilizes air damper units which are operated sothat the system opposes anticipated zone temperature changes out of theno load band.

In one preferred air conditioning system employing the invention, aforced flow of air is circulated in the system, heated and/or cooled bythe equipment and flowed through multiple, controlled zones. Theproportions of heated and/or cooled air flowing to any given zone aredetermined by a zone damper unit which is operated according to thecommand signal produced from the sensed zone temperature conditions. Inaddition, an atmospheric air damper unit is provided for controlling thequantity of atmospheric air entering the system. The atmospheric air isused for ventilating the zones and for cooling the zones to the extentpossible. The atmospheric air damper unit is controlled by the commandsignal from a controlling zone.

The zone damper units are operated through a third command signal valuerange at least part of which is between the first and second commandsignal value ranges. The command signal is configured so that the zonedamper units of a zone or zones which are neither the hottest nor thecoolest are operated to anticipate changing zone load conditionsreflected by the command signal from such zones. By anticipating thechanging conditions in these zones, they tend not to load air conditionequipment being operated by another controlling zone.

In particular, a control system constructed according to one preferredform of the invention is conditioned so that when the zone commandsignal values shift from one of the first or second value ranges towardthe other value range the damper units controlled by the command signalare operated to anticipate command signals in the other value rangewhile the zone temperature is in the no load band. The anticipatorydamper unit operation thus tends to maintain the zone temperature in theno load band without loading the equipment.

In one preferred form of the invention, the command signal configurationincludes a "no load" band region occurring at two different commandsignal values. The dual no load band command signal regions are bothbetween the command signal value ranges where the heating and coolingequipment operate. The third command signal value range within which thedamper units are operable is disposed at least partly between the duallevel no load band regions of the command signal. The command signalvalue at which the no load band region occurs is at a first level whenthe zone temperature is rising from the signal value range where theheating equipment operates and at a second level when the zonetemperature is dropping from the signal value range where the coolingequipment operates.

In order to produce the dual level no load band command signal a commandfunction generator is provided with a level shifting arrangement whichalters the operation of the command signal modifier in response todetected command signal values.

Other features and advantages of the invention will become apparent fromthe following description of a preferred embodiment made in reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an air conditioning system constructedaccording to the invention;

FIG. 2 is a schematic diagram of part of an air conditioning controlsystem embodying the invention;

FIG. 3 is a graphic representation of a configuration of a commandsignal voltage versus sensed zone temperature produced in a systemembodying the invention;

FIG. 4 schematically illustrates a command function generatorconstructed according to the invention for producing the command signalof FIG. 3;

FIG. 5 is a graphic representation of another command signalconfiguration; and

FIG. 6 is a schematic representation of a command function generator forproducing the command signal configuration of FIG. 5.

DESCRIPTION OF A PREFERRED EMBODIMENT

An air conditioning system 10 constructed according to a preferredembodiment of the invention is schematically illustrated in FIG. 1. Thesystem 10 provides conditioned air to three separate zones of a multiplezone building, which itself is not illustrated. The zones are referredto as zone 1, zone 2, and zone 3, and only zone 3 is illustratedschematically. The system 10 includes an air circulating duct network12, a blower 14 for providing a forced flow of air through the ductnetwork, air heating equipment indicated by the reference character 16and air cooling equipment 18, both of which have portions disposedwithin the duct network for heating or cooling air flowing through thenetwork, and a control system generally indicated by the referencecharacter 20 which governs operation of the system 10.

The system 10 is, for the most part, schematically illustrated and isdescribed only briefly. Many components of the system 10 are shown anddescribed in greater detail in the above-referenced U.S. patents towhich reference should be made for additional details. Departures in thesystem disclosed here from those of the referenced patents will beapparent from the following description.

The heating equipment can be of any suitable or conventionalconstruction but for the purpose of this description is considered to beconstructed from a plurality of electrical resistance heaters which areoperated in stages to govern the amount of heat transferred to the airin the system.

The air in the system is preferably mechanically chilled bycompressor-condensor-evaporator refrigeration equipment, not shown. Thecooling equipment is operable in stages to govern the amount of heatabsorbed from the air in the system. One or more evaporators of therefrigeration equipment are disposed in the duct for cooling the systemair.

The air duct network 12 comprises an air delivery duct system 22 fordirecting air from the blower 14 to the respective zones through theheating and cooling units 16, 18, respectively, a return air duct system24, only partly shown, for receiving air exhausted from the zones, and aventilation system 26 by which atmospheric air is admitted to the system10 while a corresponding amount of air from the return air duct system24 is exhausted from the system. The system 10 is of a type known as aconstant volume system in that a constant flow rate of air continuouslycirculates in the system and each zone is continuously provided with anunvarying flow rate of air.

The delivery duct system 22 comprises a blower plenum section 30 inwhich the heating and cooling units 16, 18, are disposed so that airmoving through the plenum 30 towards the zones passes across either theheating unit or the cooling unit, a zone damper section 32 at thedischarge side of the heating and cooling units, and three dischargeducts 34 (only one of which is partially shown) for directing air fromthe damper section 32 to each associated respective zone.

The damper section 32 includes three actuable damper pairs (not shown),one pair for each zone. The damper pairs are actuated by respective zonedamper control units 36, 38, 40 in accordance with temperaturerequirements of associated zones. The damper pairs for each zone enablecomplementary dampering of air flowing to that zone from the heatingunit 16 and the cooling unit 18. The damper pair for each zone has onelimit position in which all of the air flowing to the zone passes acrossthe heating unit, a second limit position in which all of the airflowing to the zone passes across the cooling unit, and intermediatepositions in which the flow of air to the zone consists of a mixture ofair which has passed across the heating unit and the cooling unit, withthe proportions of the mixture being determined by the position of thedamper pair.

The return duct system 24 comprises zone exhaust branches 42 (only oneof which illustrated in connection with zone 3) communicating each zoneto a main return duct 44 which directs the combined zone exhaust airflows to the ventilating system 26.

The ventilating system 26 comprises an atmospheric air intake duct 50through which atmospheric air is introduced into the system 10, anexhaust duct 52 through which air from the return duct 44 is exhaustedto atmosphere from the system, and a dampering arrangement forcontrolling the flow of air through the intake and exhaust ducts 50, 52.

The control system 20 governs operation of the heating and cooling units16, 18, the zone damper control units 36, 38 and 40, and the dampercontrol unit 60 in response to sensed conditions of air circulating inthe system 10 as well as sensed atmospheric air conditions. The controlsystem is preferably an electrical system and includes an air heatingcontroller 70 for governing the heat transfer to system air from theheating equipment 16, an air cooling controller 72 for governing heattransfer from the system air to the cooling equipment 18, individualzone load transmitter systems 74, 76, 78 for sensing the respective zoneloads and producing temperature related command signals for governingoperation of the controllers 70, 72, and a logic unit 80 interposedbetween the load transmitter systems and the controllers.

In the preferred and illustrated embodiment, the command signals are lowamperage DC analog signals, and the heating and cooling controllers areconstructed to operate dthe heating and cooling equipment in stages inresponse to appropriate changes in command signal values transmitted tothem. The load transmitter systems are such that as the zone airtemperature rises, the magnitude of the command signal value tends toincrease positively with respect to a reference value. As the zonetemperature is reduced, the command signal value likewise tends to bereduced.

The logic unit 80 enables the control system 20 to satisfy the heatingrequirements of the coolest zone and the cooling requirements of thewarmest zone while the heating or cooling requirements of the remainingintermediate zone is satisfied by operation of its associated zonedamper control unit alone. The command signal from the warmest zone hasthe most positive voltage level, the command signal from the coolestzone has the least positive voltage level, and the command signal fromthe zone of intermediate temperature has an intermediate voltage level.The logic unit 80 is connected to the outputs of each zone loadtransmitter and functions to transmit the command signal from thewarmest zone to the cooling controller 72 via an output conductor 86 andto transmit the command signal from the coolest zone to the heatingcontroller 70 via a conductor 88. The command signal from the remainingintermediate zone (or zones, if more than three zones are present in thebuilding) is blocked by the logic unit, but remains effective to governthe positioning of the zone damper unit for that zone.

Atmospheric air is introduced in quantity to the system to effect "free"cooling of the zones by the outside air. The introduction of atmosphericair to the air conditioning system is variably controllable by thecommand signal from the warmest zone. For this purpose a conductor 90interconnects the ventilating damper control unit 60 to the logic unitoutput conductor 86.

The load transmitter systems 74, 76, 78 are all identical and thereforonly the system 78 associated with zone 3 is described. Referring toFIG. 2, the load transmitter system 78 is illustrated as including azone temperature condition sensor 92, and a command function generator94 which coact to produce a command signal configuration as illustratedby FIG. 3 of the drawings.

The zone temperature condition sensor 92 produces a signal which is thealgebraic summation of a zone air temperature signal T, a discharge ductair temperature signal DS, and a zone 3 set point temperature signal(SP). The zone air temperature signal T is preferably provided by asensing circuit including a thermistor, or equivalent element, which isdisposed in the zone. The discharge duct air temperature signal ispreferably provided by a circuit including a thermistor or otherequivalent sensing element situated in the discharge duct 34 adjacentthe zone damper section 32. The zone set point temperature signal ispreferably provided by a circuit including a manually adjustablepotentiometer which is controlled, within limits by an occupant of thezone. The zone and duct sensing circuits are schematically illustratedin FIGS. 1 and 2 and may be of any suitable or conventionalconstruction.

The command function generator 94 includes a command signal generator 96which responds to the input zone condition signals and a command signalmodifier 98 which substantially varies the command signal under certainzone temperature conditions. A typical command signal produced by thecommand function generator 94 is illustrated by FIG. 3. The commandsignal of FIG. 3 is illustrated in terms of voltage and sensed zonetemperature and, in the illustrated embodiment, varies between limits of2 and 22 volts DC.

The operation of the heating and cooling equipment and damper units ascontrolled by zone 3 is graphically demonstrable from FIG. 3 of thedrawings. The heating and cooling equipment operate, respectively, infirst and second command signal voltage value ranges approximatelyindicated by the line segments A-B (heating) and E-F (cooling) Thedamper units operate in a third voltage value range approximatelyindicated between the line segment ends B-E. A no load band region ofthe command signal is approximately indicated by the line segment C-D.

Assuming zone 3 is at a lower sensed temperature than either of theother zones, the zone 3 command signal value varies between, say, 2 and10 volts; the heating equipment is modulated or operated in stages bythe heating controller to respond to the zone 3 command signal in thefirst value range.

When the zone 3 sensed temperature conditions rise sufficiently (e.g.,to a level between 68° F. and 70° F.,) the heating equipment is nolonger operated and the zone damper units are operated to progressivelyreduce the proportion of the air entering zone 3 which has passed acrossair heaters. When the command signal reaches the location indicated at C(12volts) the zone damper unit is conditioned to introduce equalproportions of air which has passed the air heaters and air coolers.

At this juncture the zone temperature is about 70° F. and the commandsignal enters the no load band region (line segment C-D) during whichthe sensed zone temperature may vary widely without materially changingthe command signal value. This region extends equally from each side ofthe zone set point temperature (73° F. in the illustration) and enablesthe zone temperature to float through a six degree range without anyheating or cooling equipment being operated.

Further sensed zone temperature increases result in the command signalincreasing along the line segment D-E during which the zone damper unitis operated to progressively increase the proportion of zone air whichhas passed the air cooling equipment. At the same time, assuming thatzone 3 is the warmest zone, the atmospheric air damper unit begins to beprogressively opened toward its maximum open position. This permits zone3 to be cooled by atmospheric air.

If the atmospheric air temperature increases to a predetermined levelthe atmospheric air damper unit is abruptly operated to its minimum openposition.

The cooling equipment is operated in the command signal value rangeindicated by the line segment E-F. Preferably multiple air coolers areoperated in stages and/or modulated to maintain the zone temperaturewithin limits.

A command function generator circuit capable of producing the commandsignal of FIG. 3 is illustrated by FIG. 4. The command signal generatorincludes a pair of signal amplifiers 100, 102 having their invertinginputs connected to respective reference voltage sources 104, 106, theirnoninverting inputs connected to the zone sensor 92 and their outputsconnected to a high input impedance buffer amplifier 108 via the commandsignal modifier 98. The command signal generator 96 is formed by anamplifier 120, a zone temperature condition signal source 122 connectedto the inverting input of the amplifier 120 and a reference source 124connected to the noninverting amplifier input. A negative feedbackresistor 126 is connected between the amplifier output and invertinginput to stabilize the amplifier operation.

The zone signal source 122 produces negative-going signals in responseto sensed positive-going zone temperatures so that the command signallevel output by the amplifier 120 varies directly with sensed zonetemperature changes.

The command signal modifier 98 is formed by an amplifier 130 which isrendered effective to suppress operation of the amplifier 120 undercertain conditions. The amplifier 130 has its noninverting inputconnected to the output of the amplifier 120, its inverting inputconnected to the reference source 124 and its output connected to theinverting input of the amplifier 120 via a bandwidth controllingresistor 132.

When the zone temperature is low, but rising, the amplifier 120 producesa rising low level command signal in the heating equipment operatingvalue range (line segment A-B of FIG. 3) and part of the damper unitoperating value range (line segment B-C). The suppressor amplifier 130is inactive whenever the command signal output from the amplifier 120 isalong the line segment A-C.

When the output of the amplifier 120 reaches the no load band regionlevel the suppressor amplifier 130 becomes active, in that the commandsignal value supplied to its noninverting input is sufficient to causethe suppressor amplifier to produce an output. The suppressor amplifieroutput is transmitted to the inverting input of the amplifier 120 andeffectively opposes the zone temperature signals provided by the signalsource 122. As the zone temperatures increase through the no load band(e.g., from 70° F. to 76° F.) the command signal output level from theamplifier 120 increases such a small amount that, for practicalpurposes, the command signal level is constant. Any tendency toward aninfinitesimal rise in the command signal level results in an offsettingoutput from the suppressor amplifier 130.

When the zone temperatures increase beyond the no load band (i.e., above76° F.) the suppressor amplifier 130 becomes saturated and furtherincreases in command signal level at its noninverting input do notcreate any further increases in its output level. Accordingly, operationof the amplifier 120 is not suppressed and the command signal can riseunrestrictedly along the line segments D-E and E-F in response tofurther zone temperature increases.

When sensed zone temperatures are reduced again, the command signallevel is reduced to the no load band region level at which time thesuppressor amplifier becomes active again to suppress further commandsignal level changes until the zone temperature is reduced below the noload band.

The load transmitter illustrated by FIG. 4 is described in reference toa multiple zone air conditioning control system and can be used in suchsystems as well as in single zone systems where the air heatingequipment and air cooling equipment is not simultaneously operated.

FIG. 5 illustrated the operation of another control system according tothe principles of the present invention wherein the no load temperatureband region occurs at two command signal levels. Appropriate damper unitoperation is accomplished when the command signal level is alteredbetween the dual no load band region levels. The command signalconfiguration illustrated by FIG. 5 is representative of the commandsignal produced by one load transmitter of a multiple zone system suchas that illustrated by FIG. 1 of the drawings. The command signalconfiguration illustrated by FIG. 5 is particularly well adapted for usein multiple zone systems because operation of the damper units iseffected in anticipation of changing zone load conditions and thus thezone being controlled tends to remain in the no load temperature bandover longer periods of time.

Operation of a dual no load band zone according to the invention is bestdescribed in reference to FIG. 5. As before, the heating and coolingequipment operate, respectively, in first and second command voltagevalue ranges (A-B and F-G). The damper units within a third commandsignal voltage value range between the first and second value ranges(i.e., between B and R in FIG. 5). The dual no load band regions aredefined by the line segments D-E and I-J.

When the sensed zone temperature is relatively low compared to set pointtemperature and rising, the command signal level ranges upwardly fromits minimum of 2 volts through a heating equipment operating value rangeindicated the line segment A-B of FIG. 5. While the command signal levelis varying on the line segment A-B, the heating equipment is cycled onand off, staged or modulated (depending on the air conditioning system).

When the zone temperature level increases above 68° F., the commandsignal value increases into the third value range where the damper unitsare operable. Initially the signal level is in the region indicated bythe line segment B-C of FIG. 5. In the illustrated embodiment the linesegment B-C terminates at the midrange of the command signal, i.e., 12volts. As the command signal increases from B to C, the zone damper unit36 is repositioned from a condition in which all of the air flowing tothe zone passes across the air heating equipment to a condition wherethe zone damper unit is conditioned so that half of the air flowing tothe zone passes the heating equipment and half of the air flowing to thezone passes the cooling equipment (command signal voltage at 12 volts).In other words, at the command signal midrange the zone damper unit isconditioned more or less neutrally so that the zone does not tend to beheated or cooled as a result of the zone damper positioning.

As the sensed zone temperature continues to increase the command signalincreases through the value region indicated by the line segment C-D andthe zone damper unit is progressively conditioned to proportion more andmore of the air entering the zone across the air cooling equipment. Whenthe command signal reaches the level indicated at D none of the airflowing to the zone has passed across the air heating equipment and,accordingly, if any heating equipment is being operated in response tocommand signals from other zones, the zone whose command signal isillustrated by FIG. 5 does not receive any heated air and does nottherefore contribute to the system heating load.

The damper units are conditioned for maximum cooling effect in the zone(as described) in anticipation of the zone temperature increasing. Whenthe sensed zone temperature does increase, the command signal movesthrough the no load band region indicated by the line segment D-E. Inthe illustrated embodiments of the invention the no load band regionpreferably has a voltage level at around 131/2 volts (i.e., 1.5 voltshigher than the midrange level of the command signal) and the no loadband voltage is virtually constant. When the command signal is at thelevel indicated by the line segment D-E in FIG. 5, no operation of thedamper units or the heating or cooling equipment occurs as a result ofthat command signal. Zone 3 is accordingly free to "float" in the noload band, which is approximately 6° F. wide, with the damper unitsalready conditioned so that increases in zone temperature are opposed.

Further increases of the zone temperature increase the command signallevel into the operating value range indicated by the line segment F-G.If the atmospheric air temperature is below a predetermined temperature,the atmospheric air damper unit can be modulated toward its fully openedposition by the command signal. In addition, the cooling equipment isoperated by the command signal values in the F-G value range. One ormore stages of air cooling equipment are operated (depending on commandsignal values) to direct mechanically chilled air to the zone inquestion. When atmospheric air temperature increases above thepredetermined temperature, the atmospheric air damper unit is operatedto its minimum open position.

As zone temperature is again reduced the command signal level is reducedinto the zone damper unit operating voltage range (line segment F-E-H-I)before the second no load band region level is encountered. In thisrange of command signals air cooling equipment operation isdiscontinued. The atmospheric air damper unit, assuming the atmosphericair temperature is sufficiently low, is operated to its minimum openposition by the command signal. The zone damper unit is progressivelyactuated so that at the command signal midrange the zone damper unit isagain in its "neutral" condition. Further reduction in sensed zonetemperature causes the command signal to be reduced to the levelindicated at I during which the zone damper unit is progressivelyconditioned so that more and more of the air flowing to the zone passesthe heating equipment and less and less zone air passes the coolingequipment.

The command signal next moves across the second no load band regionindicated by the line segment I-J which, as noted, is below the midpointof the command signal range and preferably around 10.0 to 10.5 volts.The second no load region is offset slightly from the first region butthe zone temperature no load band is still about 6° F. wide. Since thezone damper units are conditioned for heating the zone before the secondno load band is encountered, reduced zone temperature is anticipated bythe system thus tending to maintain the zone in the no load band.Moreover, the zone whose command signal is in this no load band regiondoes not participate in the system cooling load.

The configuration of the command signal illustrated by FIG. 6 resemblesa hysteresis loop. Controlling operation of the system in accordancewith a command signal configuration of the character illustrated by FIG.6 is particularly advantageous because the sense direction of change ofthe command signal into either no load region of the command signalconfiguration causes the damper units to be operated to anticipate theload being applied to the zone (heating or cooling). The zone dampersare thus operated to minimize the zone load to the extent possibleduring each no load band region.

A control function generator for producing a command signal of thecharacter illustrated by FIG. 5 is schematically illustrated by FIG. 6.The control function generator includes a command signal generatorgenerally indicated at 190 and a command signal modifier generallyindicated at 192.

The command signal generator includes an amplifier 200 having itsinverting input terminal connected to a suitable zone temperaturesensor, like the sensor 122, referred to above, and its noninvertinginputs connected to a fixed reference voltage source schematicallyindicated at 204. A negative feedback, stabilizing resistor 206 isconnected between the amplifier output and its inverting input.

The command signal modifier 192 includes command signal suppressorcircuitry which coacts with the amplifier 200 to provide the no loadband region of the command signal configuration and a no load bandregion level shifter circuit which coacts with the suppressor circuitryto control the command signal value at which the no load band regionoccurs. The command signal circuitry comprises a suppressor amplifierlike the suppressor amplifier described in reference FIG. 4 in that whenthe command signal output from the amplifier 200 is sufficient to causecontrolled operation of the suppressor amplifier, further changes in thecommand signal level are suppressed throughout the no load band of thezone. The suppressor amplifier 210 having its noninverting inputterminal connected to the output of the amplifier 200 and its outputterminal connected to the inverting input of the amplifier 200 via aresistor 214. The inverting input terminal of the amplifier 210 isconnected to the reference source 204 via a resistor 212.

When the command signal output from the amplifier 200 is in the heatingequipment operating range, its value is low enough with respect to thevoltage at the suppressor amplifier inverting input that the suppressoramplifier is inactive and thereafter has no effect on the amplifier 200.

When the output from the amplifier 200 increases sufficiently withrespect to the voltage level at the suppressor amplifier invertinginput, the suppressor amplifier is rendered conductive and effective tosuppress the output from the amplifier 200. The suppressor amplifieroutput is connected to the input of the amplifier 200 so that anytendency of the amplifier 200 output to increase is suppressed from theoutput of the amplifier 210.

The suppressor amplifier remains effective until it saturates, i.e. whenincreases in its input signal value do not cause corresponding increasesin its output values. When the suppressor amplifier 210 saturates, thecommand signal output from the amplifier 200 can again increase assensed zone temperatures increase.

The no load band region level shifter circuit is effective to alter thecommand signal voltage level at which the suppressor amplifier 210 isrendered effective. The level shifter includes a switching amplifier 216operable between a high output state in which the no load band region isat a relatively high command voltage level, and a low output statewherein the no load band region occurs at a lower command signal value.The switching amplifier has its inverting input connected to the commandsignal output from the amplifier 200 and its noninverting inputconnected to the fixed voltage reference 204 via resistors 220 and 212.The switching amplifier output is connected to the inverting input ofthe suppressor amplifier 210 through resistors 218, 220. Positivefeedback is provided for the amplifier 216 from its output through theresistor 218 to its noninverting input.

When the command signal values are low, i.e. in the heating equipmentoperating value range, the switching amplifier output is at its highstate or level. The reference voltage level at the noninverting input ofthe switching amplifier is sufficiently great relative to the low levelcommand signal value that the amplifier produces a high level outputsignal. The output from the switching amplifier 216 is maximized becauseof positive feedback provided through the resistor 218. The high outputstate of the switching amplifier 216 raises the voltage level at theinverting input of the suppressor amplifier 210, thus raising thecommand signal voltage level required for the suppressor amplifier 210to be rendered effective. In the illustrated embodiment the output fromthe amplifier 216 in its high state is such that the suppressoramplifier 210 is maintained inactive until the command signal voltage isaround 13.5 or 14 volts.

When the command signal voltage level is high, i.e., in the coolingequipment operating value range, the switching amplifier output is inits low state, or level. The high command signal inverting input of theswitching amplifier 216 is sufficiently great relative to the voltage atthe noninverting input that the output from the switching amplifier isreduced to a low level. This has the effect of reducing the magnitude ofthe voltage level at the suppressor amplifier inverting input thusenabling the suppressor amplifier 210 to be rendered effective at acommand signal voltage level which is comparatively low. In theillustrated embodiment of the invention when the switching amplifier 216is operated to its low output state the command signal voltage levelrequired to operate the suppressor amplifier is reduced to about 10 to10.6 volts.

The amplifiers referred to in connection with FIGS. 4 and 6 are allconventional operational amplifiers and therefore have not beenillustrated in detail. The amplifiers preferably form parts of asemiconductor device providing four such amplifiers and known as a quadop amp. These are commercially available from Motorola (M.C. 3403) andNational Semiconductor (LM324: LM 2902).

While different embodiments of the present invention have beenillustrated and described in considerable detail, the invention is notto be considered limited to the precise constructions shown. Forexample, if desired, the control system could be modified so that thedamper unit operation occurs within the no load zone temperature band.This can be accomplished by providing a command signal configurationhaving a dual level no load band region formed by a relatively lowcommand signal value section contiguous with the heating equipmentoperating value range, a high command signal value section contiguouswith the cooling equipment operating value range and an intermediateramp-like section in the vicinity of the zone set point temperaturealong which the damper units are modulated. Various adaptations,modifications and uses of the invention may occur to those skilled inthe art to which the invention relates and the intention is to cover allsuch adaptations, modifications and uses which fall within the scope orspirit of the appended claims.

What is claimed:
 1. In an air conditioning system having zone heatingequipment, zone cooling equipment and a control system for governingoperation of the heating and cooling equipment in response to electricanalog zone temperature condition responsive signals from a zone sensor,the improvement comprising:a signal amplifier having one input connectedto the zone sensor and its output connected for controlling the heatingand cooling equipment, said signal amplifier producing a command signalwhose values vary in accordance with changes in sensed zone temperatureconditions above and below a no load band of zone temperatures; acommand signal modifier means comprising a suppressor amplifier havingone input connected to the signal amplifier output and its outputconnected to said one signal amplifier input; and a reference voltagesource connected to the other inputs of said signal amplifier and saidsuppressor amplifier; said suppressor amplifier rendered effective by apredetermined command signal value indicative of a sensed zonetemperature in said no load band, said suppressor amplifier output, wheneffective, substantially preventing the signal amplifier output fromvarying from said predetermined command signal value in response tovariations in sensed zone temperature within said no load band; saidsuppressor amplifier output ineffective to prevent command signalvariations in response to variations in sensed zone temperatures outsideof said no load band.
 2. A system as claimed in claim 1 wherein saidcommand signal modifier further includes bistable switching amplifiermeans having its output connected to said other suppressor amplifierinput, said switching amplifier operable between first and second outputstates to shift the command signal values required to render saidsuppressor amplifier effective between first and second predeterminedrespective command signal levels.
 3. A system as claimed in claim 2wherein said switching amplifier means has an input connected to saidsignal amplifier output and is operated to one of its output states inresponse to command signals indicative of zone temperatures apredetermined level above said no load band, said switching amplifiermeans operated to its other output state in response to command signalsindicative of zone temperatures a predetermind level below said no loadband.